can: j1939: j1939_xtp_rx_dat_one(): use separate pointer for session skb control buffer

In the j1939_xtp_rx_dat_one() function, there are 2 variables (skb and
se_skb) holding a skb. The control buffer of the skbs is accessed one
after the other, but using the same "skcb" variable.

To avoid confusion introduce a new variable "se_skcb" to access the
se_skb's control buffer as done in the rest of this file, too.

Cc: Robin van der Gracht <robin@protonic.nl>
Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20210616102811.2449426-6-mkl@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>